thin_film_bsdf separately fails compile due to undefined variables

[kwokcb]

An edge case when trying to create shader code with just this node:

<?xml version="1.0"?>
<materialx version="1.38">
  <thin_film_bsdf name="thin_film_bsdf" type="BSDF">
    <input name="thickness" type="float" value="550" unittype="distance" unit="nanometer" />
    <input name="ior" type="float" value="1.5" />
    <output name="out" type="BSDF" />
  </thin_film_bsdf>
  <surface name="shader_thin_film_bsdf_out" type="surfaceshader">
    <input name="bsdf" type="BSDF" nodename="thin_film_bsdf" />
  </surface>
  <surfacematerial name="material_thin_film_bsdf_out" type="material">
    <input name="surfaceshader" type="surfaceshader" nodename="shader_thin_film_bsdf_out" />
  </surfacematerial>
</materialx>

Gives this GLSL error when accessing thin_film_bsdf_out.response.
Error also occurs for OSL so assume it's a generic code gen issue.

0(923) : error C1503: undefined variable "thin_film_bsdf_out"
0(932) : error C1503: undefined variable "thin_film_bsdf_out"
`` 
[thin_film_bsdf_ps.glsl.txt](https://github.com/AcademySoftwareFoundation/MaterialX/files/10428633/thin_film_bsdf_ps.glsl.txt)

[jstone-lucasfilm]

This issue has been addressed in #1413 for the in-progress v1.39 of MaterialX, so let's go ahead and mark this as resolved.